Fuel-leak detector and safety system

ABSTRACT

A safety control system for detecting and stopping leakage of fuel through the fuel-valve in burner fully automatic installations subject to such leakage problems because of the presence of fuel under pressure at the fuel-valve for periods of time when the burner is not actually lighted. The system includes a lead-detector, such as a pressure switch, located downstream of the main fuel-valve for closing an auxiliary fuel-valve installed in tandem with the main valve, so that when leakage through the main valve occurs, the auxiliary valve closes and immediately stops the leak. It also trips a lockout circuit that prevents reignition of the burner. A discriminator device is employed for nullifying the action of the leak-detector on normal flow of fuel to the burner nozzle. While the leak safety of the present invention immediately stops the leak by closing the auxiliary fuel-valve, it permits the master burner control to progress through its normal cycle of operations during both its shut-down and start-up periods to the point just before ignition takes place, so that any fuel that may have leaked will be purged during the purge periods of the burner cycle, thereby providing a safeguard against ignition of the leaked fuel when the burner is restarted. A leak alarm is provided to signal when the leakdetector senses a fuel leak, and a separate manual reset device is provided so that the burner cannot be restarted accidentally following what might be erroneously thought to be a routine flame-out.

United States Patent 1 Faulkner m in, W73

Primary ExaminerCarroll B. Dority, Jr. Attorney-Steward and Steward, Merrill F. Steward and Donald T. Steward [5 7 ABSTRACT A safety control system for detecting and stopping leakage of fuel through the fuel-valve in burner fully automatic installations subject to such leakage problems because of the presence of fuel under pressure at the fuel'valve for periods of time when the burner is not actually lighted. The system includes a lead-detector, such as a pressure switch, located downstream of the main fuel-valve for closing an auxiliary fuel-valve installed in tandem with the main valve, so that when leakage through the main valve occurs, the auxiliary valve closes and immediately stops the leak. It also trips a lockout circuit that prevents re-ignition of the burner. A discriminator device is employed for nullifying the action of the leak-detector on normal flow of fuel to the burner nozzle. While the leak safety of the present invention immediately stops the leak by closing the auxiliary fuel-valve, it permits the master burner control to progress through its normal cycle of operations during 15 Claims, 4 Drawing Figures FUEL-LEAK DETECTOR AND SAFETY SYSTEM BACKGROUND OF THE INVENTION The present invention relates to control systems for fully automatic fuel burners of various kinds, including gas and oil burners for furnaces and boilers, and it relates more particularly to safety devices for preventing leakage of fuel through the burner when it is not lighted.

In automatic commercial and industrial heating systems, the intermittent operation of the burner usually takes place with no-one in attendance at all. Consequently, if the main fuel-valve leaks or does not function properly when the burner should be off, fuel will leak through the burner nozzle into the combustion chamber of the furnace, heater or the like. Leakages of this kind occur most often in installations where the fuel to be burned is continuously supplied under pressure to the main fuel-valve. However, they can also occur in installations which have long preand postignition cycles during which the fuel pump is running for a considerable length of time while the main valve is closed. The resulting danger of exploding the fuel in the furnace when normal ignition takes place has been a long-standing problem in The heating industry. A potentially explosive amount of unburned fuel can readily accumulate in the combustion chamber if fuel leaks through the main fuel-valve while the burner is in a standby condition, or during only the start-up and shutdown portions of the burner cycle.

The imminence of the danger is punctuated by the following quotations from the Jan. 1, 1970 issue of Honeywell Flame Tips Vol. 5 No. 6:

A cubic foot of natural gas (or about oz. of fuel oil) properly mixed and vaporized and confined, has the explosive equivalent of a stick of dynamite. A burner firing at full input into a properly designed and sized combustion chamber takes about 8 seconds to fill the chamber. If light-off occurs with the chamber A or less full (about 2.66 seconds at high fire) the result is a puff. if light-off occurs with the chamber more than A: full, the result is a substantial explosion."

In order to reduce the possibility of an explosion due to accumulation of fuel in the tire box, regulatory authorities for the industry have established certain safety precautions. One such precaution is the requirement that the blower run for a predetermined length of time during the start-up cycle before the ignition source is turned on and the fuel valve opened, in order to try to purge the combustion chamber of fuel before ignition takes place. This is referred to as the pro-purge period. The trend of modern automatic burner control designs has been toward longer pre-purge periods, with the hope of diluting any explosive mixture which may be in the combustion chamber and of flushing so much air through it that all of the combustible mixture will be carried out the stack before ignition takes place. There is, however, a serious difficulty in this approach to the problem, which has not been generally appreciated, but which is nevertheless inherent in it.

For example, if the fuel valve is leaking during the pro-purge period, a large quantity of fuel can be injected into the combustion chamber along with the purging air, thereby increasing, rather than decreasing, the hazard that existed at the start of the pre-purge period. Similarly, if the leak occurs during the post-purge period while the burner is being shut-down upon satisfaction of the requirement for heat, more fuel may be pumped into the combustion chamber during a prolonged post-purge period than during a shorter one. Furthermore, fuel oil that leaks through the burner nozzle during such purge periods impinges or condenses on the walls of the combustion chamber and cannot be flushed out by the air blown through the combustion chamber even during the longest prepurge period that is possible. Consequently, where the fuel valve is leaking, longer purge periods can cause a more disastrous explosion, instead of avoiding one. This is true whether the fuel is natural gas or fuel oil, but more particularly in the case of oil.

It has been proposed heretofore in connection with semi-automatic burners to provide a fuel-leak detector which relies on a continuously burning pilot flame to ignite fuel sensed downstream of the main fuel-valve when the main valve is scheduled to be closed. The resulting flame is detected by a flame detector which then closes an auxiliary valve in the fuel supply line in order to stop the leak. One of the serious disadvantages of such a system is that in order to restart the burner, the standing pilot must be lighted manually before the leak detector is operable, and if the combustion chamber contains a sufficient volume of combustible air-fuel mixture, a disastrous explosion can occur. Furthermore, there is no safeguard against leakage of fuel into the combustion chamber if the standing pilot flamesout, because the leak detector is completely dependent upon the flame detector.

As a practical matter, moreover, such a leak detector is not approved for use by the industry in fully automatic installations, because of its reliance on a standing pilot which is not allowed in unattended (i.e. fully automatic) burner installations. The reasons that standing pilots are not approved for automatic operation are that the fuel leaking into the combustion chamber either is ignited by the pilot and burns, or creates a fuelrich mixture causing the pilot to flame-out. As pointed out in the United States patent to Brown US. Pat. No. 3,223,138, continued burning of the leaked fuel in an unattended system can result in a disastrous overheating of the unit, boiler explosions and the like. Pilot flame-out in an unattended system, on the other hand, can result in equally disastrous consequences if, for example, an attempt is made to relight the burner manually before the combustion chamber is cleared of all fuel. Consequently, standing-pilot burners, in which a pilot flame burns continuously in the combustion chamber of the heating unit, are not approved for fully automatic, unattended systems.

The term automatic fuel-burner is used in the pres ent disclosure and claims in the same manner that it is used throughout the industry. Burners or control units that are approved forcompletely unattended operation for extended periods of time are considered to be automatic. But those which rely on a continuously burning flame or pilot for ignition purposes are not so approved and are therefore not considered automatic.

in industrial installations using heavy Bunker C fuel oil, the fuel valve may leak or stick in a partially or fully open position while the burner is unattended, usually during an overnight shut-down. Automatic controls have been designed to prevent ignition of the burner when such a failure occurs, but to my knowledge no provision has ever been made for stopping the flow of fuel oil into the combustion chamber following a fuelvalve failure, in a completely automatic control system. The result is that unburned fuel oil builds up inside the combustion chamber and may even flood the fire box completely. By the time the trouble is discovered, there is in all probability a collosal mess of congealed Bunker C oil, which must be pumped or otherwise removed with great diffulty from the combustion chamber. Not only is this cleaning process time-consuming and expensive, but the resulting downtime of the furnace, and possibly of a whole plant, is extremely costly. It is important to note in this connection that insurance companies do not usually insure against losses due to damaged equipment and downtime unless an explosion takes place. Consequently, the cost of removing fuel-oil from a furnace, cleaning up and reconditioning the burner following fuel-flooding, which is not infrequent, is not paid for by the insurance company. Furthermore, downtime of plant and equipment in such instances cannot be insured against.

There is also the possibility that a smaller, but still potentially explosive, quantity of fuel may leak into the combustion chamber unnoticed during or after an automatic shut-down for one reason or another. Should the attendant then unthinkingly reset the controls in order to restart the burner, ignition of the accumulation of gas or oil in the combustion chamber may occur with disastrous results. Explosions of this kind occur only too often, with serious injury to personnel and destruction of property.

In addition to lengthening the pre-purge and postpurge periods in the burner operating cycle which, however, as indicated hereinbefore may increase, rather than decrease the hazard, other safety devices have been employed with only limited success. For example, dual main fuel-valves are sometimes installed so that if one leaks, the other will still hold. The disadvantage of such a system is that if one of the valves fails, there is no way of knowing that it is not working until the other valve sticks open or leaks. While the length of time for both valves to fail may be longer than where only one is used, the results are just as disastrous when both valves have failed. Furthermore, since both valves open and close together during each burner cycle, there is not even any assurance that one will last any longer than the other.

The hereinbefore-mentioned patent to Brown U.S. Pat. No. 3,223,138 discloses a safety for preventing restarting of a constantly attended burner when the fuelvalve leaks. In this case, an inaccessible disabling relay is provided for locking out the controls, so that the attendant cannot inadvertently restart the burner, if the fuel-valve is leaking, without first calling his supervisor or a burner maintenance man capable of diagnosing the trouble and repairing or replacing the valve. However, while the safety system of the Brown patent prevents inadvertent start-up of the burner when a potentially dangerous condition exists, it does not take care of the source of the trouble insofar as automatic controls are concerned, namely the continued leakage of fuel into the combustion chamber after the burner has been shut-down. Consequently, the solution provided in this patent is of little value for automatic, or in other words, unattended burner operation.

Thus, if some time elapses after shut-down due to a leaking fuel valve and before the difficulty is discovered, a considerable amount of fuel may have leaked into the fire box by the time the trouble is discovered.

The maintenance or servicing crew will accordingly have a back-breaking task of cleaning up the mess, particularly if the fuel is Bunker C oil. It is not surprising, therefore, that the approach to the problem disclosed in the Brown patent has not been adopted in, nor is it applicable to, fully automatic burner controls. Instead, longer pre-purge and postpurge periods have been resorted to by some of the leading burner control manufacturers in the country.

The principal objects of the present invention are to:

l. prevent fuel explosions by attacking the trouble at its source, namely the leaking fuel-valve, by automatically monitoring, detecting and stopping the leak immediately,

2. lock-out the burner controls independently of other safety lock-outs in the system whenever a fuel-leak occurs through the fuel-valve, thereby permitting the blower or forced-draft means to purge the combustion chamber of dangerous quantities of combustible air-fuel gases, and

3. provide a leak-alarm which is actuated only when the fuel-valve leaks or does not close properly, so that the trouble is pin-pointed immediately and can be repaired without having to dismantle and check other possible causes of trouble.

Another important object of the invention is to eliminate the necessity of removing quantities of raw fuel which may flood the combustion space for the burner due to failure of the fuel valve to close. This objective, however, goes hand-in-hand with that of preventing explosions by immediately detecting and stopping the leak through the fuel-valve.

SUMMARY OF THE INVENTION In general, the invention resides in monitoring the presence of fuel downstream of the main fuel-valve both during standby periods of operation of the burner and during the portions of the burner cycle when the main fuel-valve should be closed, so that if fuel leaks through the ostensibly closed main fuel-valve, a control lock-out will actuate to close an auxiliary fuel valve which stops the leak and at the same time prevents operation of the burner until the faulty valve is repaired or replaced. To this end, a leak-detector, such as a pressure-actuated switch in the controls for the auxiliary valve that is capable of functioning whether or not a source of ignition has been established, is installed with its fuel sensor in the fuel-supply line between the main valve and the burner nozzle. The auxiliary fuelvalve controls include a dicriminator device for nullifying the action of the leak-detector when it senses the flow of fuel during a normal, scheduled opening of the main fuel-valve, thereby ensuring regular operation of the burner while monitoring the system for fuel leaking through the burner when it is not ignited. The discriminator may be a delayed-action relay, a timer switch or any device capable of preventing the closing of the auxiliary fuel-valve and of co-ordinating with the scheduled opening of the main fuel-valve during the burner operating sequence, so that when the leak-detector senses the flow of fuel to the burner upon normal opening of the main valve, the auxiliary fuel-valve does not close and prematurely shut down the burner DESCRIPTION OF THE PREFERRED EMBODIMENTS The advantages of the invention will become more apparent from the description hereinafter of certain embodiments of the invention, illustrated in the accompanying drawings, wherein FIG. l is an electromechanical schematic view of a fuel-oil burner installation and control system therefor that is more-or-less typical of the prior art;

FlG. 2 is a view similar to FIG. ll, but showing the leak-detector apparatus and controls therefor of the present invention incorporated therein;

FIG. 3 is a bar graph showing the time relationship of the operation of the contacts of a timer which forms part of the control apparatus of FIGS. 1 and 2; and

H6. 4 is an electro-mecnanical schematic view of the leak-detector apparatus and controls of the present invention incorporated in an addon unit for an existing burner installation, such as that illustrated in FIG. 11.

in the somewhat simplified oil-burner installation of the prior art illustrated in FlG. l, the burner has the usual blower l2, ignition electrode l4, burner nozzle 16, flame-detecting photo-electric cell 18 and gas-pilot burner 1'19. Oil is continuously supplied under pressure to a main fuel-valve 2t? through a supply line 22. The switch contacts in the control and power circuits are shown in the positions assumed when the burner is shut down with its operating limit-switch A, running interlock-switch B, airflow-purge interlock-switch C and starting-ignition interlock-switch D, all in the open position. Electrical power is supplied to terminals 2 and 4 across the hot feed line 24 and ground 26 from a suitable power source such as a 120 volt A.C. supply. On closing the operating limit-switch A preliminary to automatic operation of burner 10, a master relay 11R is energized through a pair of normally closed lockoutswitch contacts LS-l and initially closed contacts T11 of a sequencer or timer T.

Reference should also be made to the bar graph of H6. 3 showing a detailed sequence of operations carried out by timer T during each burner cycle, commencing with the closing of the operating limit-switch A through shut-down of the burner following satisfaction of the requirement for heat. Energization of relay closes contacts ilR-ll in a power circuit to the burner blower 1 .2. for starting the pre-purge period. Closing of contacts ilR-ll also completes a circuit through initially closed timer contacts T2 and T3 to the timer T, contacts llR-Z having opened on energization of master relay llR. Blower l2 and timer T therefore start running immediately. At the same time contacts lR-L'l and llR-l both close. Contacts lR-B are located in the lockout circuit of a safety-switch having a heater LS-H, energization of which opens contacts LS-ll. Contacts 1R4 are located in the power line to an ignition transformer 28, main fuel-valve 20., a flame-detector MD and a shut-off valve 31 for the gas pilot 19.

On closing of the running interlock-switch B, a holding circuit is established for the master relay 11R through initially closed timer contacts T4 and T5, closed relay contacts lit-3 and a set of initially closed relay contacts 2R-l of a relay 2R in the flame-detector system 3t). Consequently, even though timer contacts Tl open shortly after timer T starts, master relay 1R holds in. Contacts 2R-ll are closed when its relay ER is not energized and are open when relay 2R is energized, the reverse being true of contacts ZR-Z in an alternate circuit through initially open timer contacts T6. If the flame-detector relay 2R holds in during the pre-ignition period, or if it is de-energized for a period of about one minute after ignition because of ignition or flame failure, lockout-switch heater LS-H will trip out its contacts LS-l. This in turn de-energizes master relay HR, opening the circuits through relay contacts ilR-l, including the main fuel-valve 26, which shuts down the burner. At the same time a flame-failure alarm circuit is energized on closing of another set of contacts LS-Z which are actuated simultaneously with contacts LS-l when heater LS-H is energized for a prolonged period of about sixty seconds. Lockout-switch contacts LS-i and LS-2 do not automatically release from their safey positions when they have been tripped and must be reset manually after heater LS-H has cooled. The burner is therefore locked out until it is manually restarted again.

On the other hand, provided flame-detector relay 2R does not hold in during the pre-ignition period when the flame-detector is energized through initially closed timer contacts T7, the master relay HR is kept energized by its holding circuit through relay contacts ZR-ll, or if need be for a brief time through heater LS-l-l. Power for the blower l2 and the control circuit for timer T is therefore assured through contacts illR-l, even during the short period at the start of the burner cycle when timer contacts T9 are open. Nevertheless, timer T stops when its contacts T2 open after about 20 seconds into the cycle, but restarts again when purge interlock-switch C closes following indication of a satisfactory purge. At about 53 seconds into a cycle, timer contacts T3 open, and timer T again stops until the ignition-starting, interlock-switch D closes and provides an alternate circuit through a set of flame-detector contacts ZR-S and timer contacts T8. During the normal start-up, flame-detector relay 2R will not be energized at the time contacts T3 open. Contacts 2R5 are therefore usually closed when interlock-switch D closes, so that timer T will restart in order to advance the burner cycle.

it will also be noted that if a flame-failure lockout occurs, timer T automatically recycles itself, due to the fact that contacts lit-2 close, completing the timer circuit through timer contacts Ti which, as will be seen in the timer bar graph of FIG. 3, closed almost immediately after timer T started and do not open until the end of the cycle. Contacts T9, therefore, provide a holding circuit for the timer, permitting it to recycle when the master relay ER is de-energized for any reason.

Assuming that all the switches A. through D close and that lockout does not occur, contacts Tilt close to energize the ignition circuit about 1 minute after the start of the cycle, thereby completing a circuit from the before-mentioned timer contacts T7 through timer contacts Tllll to the primary winding of ignition transformer 2%. Another circuit is also completed on closing of timer contacts Tilt) through a shut-off valve 311 in the gas line to the pilot I19, thereby turning on the gas simultaneously with energization of electrode ll i. 0n establishment of a pilot flame, timer contacts T113 close in order to energize the solenoid of main fuel-valve 23th, which thereupon opens. If a flame-out occurs, flamedetector relay 2R becomes rile-energized, reversing its contacts ZR-Il and ZR-Z, which in turn energizes heater LS-l-il due to the fact that timer contacts T5 opened at the same time that contacts Tiltll closed. However, if a flame is established and barring some other malfunction, the burner continues to operate until the demand for heat is satisfied. During the portion of the ignition period between the ignition of the gas-pilot 19 and the establishment of a flame following opening of the fuelvalve 20, power to the master relay IR is maintained without energizing heater LS-H by a circuit through timer contacts T6 which close at the same time that the igniter is energized. In a normal start-up, timer T will stop running temporarily when its contacts T3 open following establishment of the flame. The burner continues to run, however, as long as heat is called for and as long as the flame does not fail.

Upon satisfaction of the demand for heat, operating limit-switch A opens, de-energizing master relay IR and closing main fuel-valve 20, thereby extinguishing the flame. De-energization of relay 1R also closes the contacts 1R-2, completing a circuit through timer contacts T9, in order to restart timer T. The burner blower 12 continues to run through a post-purge period of about 15 seconds until timer contacts T9 open at the end of the timer cycle. Timer T also stops with the opening of its contacts T9, and the system is ready to start-up automatically when heat is again called for. The master control system shown schematically in FIG. 1 and the portion of the timing sequence therefor shown in FIG. 3 are simplified versions of those employed by Electronics Corporation of America in their control systems designated FIREYE System FP-2 Using Type 26RJ 8 Model 6070 and described in their company Bulletin CP- 9.

Referring now to FIG. 2, which shows a control system that is identical with that of FIG. 1 except that it is modified to incoporate the fuel-leak safety of the present invention, an auxiliary fuel-valve 32 is installed at the burner in tandem with the main fuel-valve 20, and a leak-detector 34 is installed in the fuel line 22 to burner nozzle 16 downstream of main fuel-valve 20. The auxiliary fuel-valve is desirably a motorized type having a motor actuator for opening the valve when power is supplied to it and having a quick-closing spring action, thereby ensuring rapid closing of the valve when its actuator is de-energized. The leakdetector 34 is shown diagrammatically to represent a pressure switch having a set of mechanically actuated contacts LD, which are connected to the power line 24 ahead of the operating limit-switch A by means of con ductors 35 and 36, and to ground 26 by means of conductors 37, 38 and 39. Conductor 37 connects one of the leak-detector contacts LD to one side of a spare set of contacts T14 of timer T, which may be conveniently employed as a discriminator for the purpose of nullifying the action of the leak-detector during the period of the burner operating cycle when the main fuel-valve is open. Conductor 38 connects the other side of contacts T14 to a control relay 1CR, which in turn is connected to ground 26 by conductor 39. Leak-detector contacts LD are normally open, closing when fuel is detected downstream of main fuel-valve 20.

Auxiliary fuel-valve 32 is connected to the power line 24 ahead of the operating limit-switch A by means of conductors 35, 36, 40 and 41, and to ground 26 by a conductor 42 common to both the fuel-valves. It will be noted that by connecting both the leak-detector 34 and auxiliary fuel-valve 32 ahead of the operating controls, these units are operable both during the burneroperating cycles and while the burner is in a stand-by condition. A holding circuit, that includes conductors 43, 44 and 45, provides an alternate circuit for control relay lCR from conductors 35 and has a manual reset switch MRS between conductors 44 and 45 for momentarily opening this holding circuit in order to deenergize control relay ICR. In addition, a leak-alarm 46 is connected from conductor 35 to the ground 26.

Leak-detector control relay lCR has four sets of contacts as follows:

lCR-l are located in the holding circuit for control relay lCR between conductors 43 and 44, and are open when lCR is de-energized;

lCR-2 are interposed in the ignition-transformer circuit between timer contact T10 and T1 1 and are closed when 1CR is de-energized;

lCR-3 are located in the leak-alarm circuit and are open when lCR is de-energized; and

1CR-4 are located in the auxiliary fuel-valve circuit between conductors 40 and 41 and are closed when lCR is de-energized.

It will be noted from FIG. 3 that the timer contacts T14 are closed at the start of the timing cycle, open shortly before the ignition transformer 28 is energized by the closing of contacts T10, and close shortly after the main fuel-valve 20 closes on de-energization of the master control relay 1R when operating control-switch A opens upon satisfaction-of the requirement for heat. As will become more apparent hereinafter, timer contacts T14 discriminate between the times when the leak-detector system should, and should not, function to shutdown or prevent operation of the burner. It will also be apparent that as long as leak-detector contacts lCR-4 are closed, auxiliary fuel-valve 32 is energized and is therefore open, whether of not the operating control is calling for heat.

Operation of the leak-detector system shown in FIG. 2 as a built-in part of the burner control system is as follows. If the main fuel-valve 20 leaks during a standby period when the burner is not running, lead-detector 34 senses the oil that leaks through the main fuel-valve 20, and its contacts LD close. Since the discriminator contacts T14 remain closed throughout the standby period of the burner, the leak-detector circuit through control relay lCR is completed upon closing of contacts LD, thereby immediately energizing control relay lCR even though the burner is in a standby condition not actually running. Consequently, all four sets of contacts of control relay lCR reverse their conditions, including 1CR-4 which open in order to de-energize the auxiliary fuel-valve circuit, thereby closing auxiliary fuel-valve 32. The leak is accordingly immediately stopped, preventing the accumulation during standby of unburned fuel in the fire box of the furnace or boiler. At the same time that auxiliary valve 32 closes, the leak-alarm circuit is energized by the closing of contacts lCR-3, immediately signalling the attendant while the burner is still in standby condition that the main fuel-valve is not functioning properly and that it should be checked and serviced. 1

If, on the other hand, no-one is on the premises at the time a leak occurs, or if the leak-alarm goes unnoticed until there is a call for heat, the burner will start its operating cycle in the usual manner when operating limitswitch A closes. Timer T will start to run as usual and will automatically sequence the start-up operations to the point at which the ignition circuit is to be energized by the closing of timer contacts T10. However, when contacts T10 close, the ignition circuit through transformer 28 is not completed because the leak-detector control-relay contacts 1CR-2 are open due to energization of TCR upon actuation of leak-detector 34 in response to the presence of fuel downstream of main fuel-valve 20. Consequently, there is no ignition spark at electrode 14, the gas-pilot valve 39 does not ignite and the burner controls are locked-out by the flamedetector system, which in this instance is the same as that shown in FIG. 1. Thus, since flame-detector relay contacts 2R-2 are open due to the fact that relay 2R is not energized when the flame fails, heater LS-l-l becomes energized when timer contacts TS open simultaneously with the closing of contacts T10. Heater LS-H will then remain continuously energized long enough to trip its lock-out switch LS-i, thereby de-energizing master relay ilR and opening contacts lR-l, 1R-3 and LR-4. There is accordingly no possibility of light-off or even of energizing the ignition transformer, following detection of a leak, and the potential danger of igniting a residue of combustible fuel in the fire box is eliminated.

it will be noted that contact lCR-l in the holding circuit for control relay lCR close when ICR is energized. Control relay lCR, therefore, remains energized until the holding circuit is broken by manual opening of the reset switch MRS. A second and completely independent lock-out of the controls is therefore provided by the leak'detector safety system of the present invention. it should also be noted that the leak-detector system will operate both during the pre-purge period and post-purge period of the burner operating cycle, as well as during the standby condition of the burner just de scribed. However, in order to prevent the leak-detector 34 from interfering with the normal operation of the burner, it is necessary to nullify its action during the portion of the burneroperating cycle when the main fuel-valve 20 should be open.

Thus, under normal operating conditions of the burner, the discriminator-timer contacts T14 open a few seconds (see FIG. 3) before the ignition tranformer 28 is energized, thereby preventing leak-detector contacts LD from completing the circuit through control relay lCR when main fuel-valve 20 opens. If desired, however, contact T14 may be set to open simultaneously with energization of the igniter, or in some installations just prior to the opening of the main fuelvalve. Upon closing of discriminator-timer contacts Tl i seconds after main fuel-valve 20 closes on shutdown, the action of leak-detector 34 becomes effective again in detecting and signalling the presence of fuel downstream of the main fuel-valve 20 during th postpurge period of operation of the burner. Should the main fuel-valve not close properly, the auxiliary fuelvalve immediately closes automatically to stop the leakage of fuel into the firebox, and the leak alarm is actuated so as to call attention to the trouble. Moreover, as mentioned hereinbefore, the burner is in effect locked-out against automatic operation on the next call for heat, but will nevertheless begin its start-up program through the prepurge period before being completely locked-out by the flame-fail safety. Under no circumstances is fuel permitted to pass downstream of the main valve while it is closed without closing the auxiliary fuel valve and simultaneously actuating a leak alarm. Furthermore, detection of undesired fuel downstream of the main valve actuates the auxiliary valve independently of the main valve to prevent prolonged leakage of fuel into the fire box while the burner is not lit.

lltl

LEAK-DETECTOR ADD-ON UNIT OF H6. 4

Reference is now had more particularly to the modification of the invention shown in FIG. 4, which illustrates an add-on unit for providing the leak-detector apparatus and control of the present invention in existing burner installations. For convenience the same reference characters are employed in designating mechanical and electrical components of the modification shown in FIG. 4 that are identical with those used in the control system of H6. 2. The addon unit consists of an auxiliary valve and leak-detector, pressure-switch assembly 5b, and an add-on control panel 52, the assembly 50 being constructed so that it can be readily installed in the fuel supply line 22 (FlG. l) downstream of main fuel-valve Zll of the existing installation. lf miniturized electronic components are employed, the add-on control panel 52 may be mounted directly on the assembly Stll, forming a self-contained unit therewith. However, panel 52 can also be installed at the existing master control panel for the burner, in which case suitable terminals should be provided at the addon panel for the auxiliary fuel-valve 32 and leakdetector 34, in addition to those shown in FlG. t for connecting the unit to the master controls.

For the most part, installation of the add-on controls with the master controls for an existing burner is simply a matter of making electrical connections from existing terminals at the master control panel to corresponding terminals provided on the leak-detector, add-on panel 52. As shown in FIG. 1 most burner control panels are provided of the terminal marked 2 located in the ground 26 and another marked 4 located in the power supply line 24. A terminal marked 3 is also usually available on the load side of the operating-control switch A. in some installations additional terminals may be available, making it unnecessary to break into an existing circuit in order to make the connections required for the leak-detector add-on unit of the present invention. For purposes of the present illustration, however, FIG. ll shows a jumper wire from points marked 5 and 5A in the conductor to the ignition transformer 2%, where two to he connections for the add-on unit can be made.

The add-on control panel 52 should therefore be provided with correspondingly numbered terminals 2, 3, 4, 5, and 5A, which can be connected to the corresponding terminals shown in FIG. ii, in order to interpose the leak-detector safety system shown in FIG. 4 into the existing master control system. As in the system shown in FlG. 2, the auxiliary valve 32 is opened on energization of its control circuit from terminal 4 through control relay contacts llCR-l of control-relay TCR. In this instance, however, llCR is located on the add-on panel 52. Contacts lCRd are initially closed and open on energization of control-relay lCR to tie-energize the auxiliary fuel-valve circuit and close the auxiliary valve 32. Control relay lCR is located in the leak-detector circuit through the contacts lLD of leak-detector 343 from terminal 43 to the ground terminal 2. Control-relay llCR again has contacts llCR-ll, lCR-Z, and lCR-3, which function in the same manner as in the system of FIG. 2.

While the add-on unit could be arranged to employ a spare set of contacts in the timer T of the master control system for isolating the leak-detector circuit while the main fuel-valve 2b is open during a normal burneroperating cycle, such timer contacts may not always be available in the existing installation. Therefore, in order to make it applicable in various different burnercontrol systems, the leak-detector add-on unit shown in FIG. 4 is provided, by way of example, with a somewhat different discriminating system from that employed in the system of FIG. 2. Thus, a discriminator relay 2CR is located in a circuit that is energized on energization of the ignition transformer, its initially closed contacts 2CR-2 being located in the leak-detector circuit and opening with energization of relay 2CR in order to prevent energization of leak-detector relay lCR during the period when the fuel valve is scheduled to be open. As indicated hereinbefore terminals and 5A of the addon panel 52 are connected at points 5 and 5A, respectively, in the ignition circuit of the existing controls (FIG. 1), the jumper wire shown in FIG. 1 of course being discarded when the connections for the add-on unit are made. Contacts lCR-2 of leak-detector relay lCR are disposed in a circuit 5-5A of the add-on unit between terminals 5 and 5A so that when lCR is energized 1CR-2 opens to interrupt the circuit to ignition transformer 28. Discrimination control-relay 2CR is connected in parallel with the igniter in a circuit from a point 60 in the circuit 5-5A through a point 62 to ground terminal 2, so that whenever power is supplied to the igniter, control-relay 2CR is energized.

Contacts 2CR-2 of the dicriminator relay are located between contacts LD and the leak-detector controlrelay lCR, such that when relay 2CR pulls in, contacts 2CR-2 open, preventing energization of leak-detector relay lCR. Discriminator relay 2CR is also provided with initially closed contacts 2CR-1 in the discriminator circuit from terminal 5 between points 60 and 62, as well as with initially open contacts 2CR-3 in a holding circuit through terminal 3, which is connected to the corresponding terminal of the master control system after operating limit-switch A (FIG. 1). On energization of relay 2CR, contacts 2CR-1 open, and simultaneously therewith contacts 2CR-3 close to hold-in relay 2CR until operating switch A opens on satisfaction of the call for heat.

It is apparent from the foregoing that when timer switch T10 closes during a normal startup of the burner, the ignition transformer 28 is energized in the conventional manner described hereinbefore in connection with the prior art controls shown in FIG. 1. However, with installation of the leak-detector add-on unit of the present invention, power for the igniter is provided through control-relay contacts 1CR-2 in the circuit 5-5A of the add-on panel 52. Consequently, when terminal 5 is energized, discriminator relay 2CR also becomes energized, opening discriminator contacts 2CR-2 in the leak-detector circuit prior to opening of main fuel-valve 20 when timer contacts T13 close. The leak detector relay lCR, therefore, does not become energized when contacts LD close on activation of leak-detector 34 by the normal flow of fuel downstream of the main valve. The burner will accordingly light-off in the usual manner and continue to burn as long as operating limit-switch A holds in.

It will be noted, moreover, that even though the igniter is de-energized following establishment of a flame by the burner 10, discriminator relay 2CR is held in throughout the burn by the closing of its contacts 2CR- 3, thereby preventing closing of the auxiliary fuel-valve 32 during normal operation of the burner. On satisfaction of the heat requirement, operating limit-switch A opens closing main fuel-valve 20 and de-energizing terminal 3. Discriminator relay 2CR, therefore, drops out, permitting leak-detector 34 to again monitor the presence of fuel downstream of main valve 20. Leakdetector contacts LD in the meantime will have opened, provided of course that main fuel-valve 20 closes properly when operating limit-switch A opens, so that control-relay lCR is not energized when contacts 2CR-2 close.

If, however, the main fuel-valve 20 does not close, contacts ID of leak-detector 34 do not open, so that control-relay lCR is energized immediately upon the closing of discriminator contacts 2CR-2. The leakdetector controls then take over as described in connection with the system shown in FIG. 2 to close auxiliary fuel-valve 32 when contacts 1CR-4 open. The leak-detector contacts lCR-2, which are located in the ignition circuit 5-5A of the add-on panel, likewise open when lCR pulls in, so that ignition cannot take place. Contacts 1CR-3 of the leak-detector relay lCR also close in order to set-off a leak alarm, while contacts lCR-l close to complete a holding circuit for relay lCR directly from power terminal 4. The burner controls are then in effect locked-out and can only be reset by manually opening reset switch MRS in the holding circuit for relay lCR, in order to let it drop out.

Similarly, during a standby period of the burner while operating limit-switch A is open, power is supplied to auxiliary fuel-valve 32 from terminal 4 through closed contacts lCR-4 to ground at terminal 2. Auxiliary fuelvalve 32 therefore remains open as long as contacts 1CR-4 are closed. Since contacts ID in the leakdetector are open when fuel is not present downstream of the main fuel-valve 20 this course being the normal condition during such intermittent standby periods when the burner is not firing control-relay lCR is not energized, and the burner controls function normally as hereinbefore described. However, if the main valve 20 leaks during standby, fuel will be detected by leak-detector 34, which will close contacts LD completing the leak-detector circuit from terminal 4 through closed contacts 2CR-2 and control relay lCR to ground at terminal 2. Control-relay lCR is therefore energized, opening contacts lCR-4 in the auxiliary fuel-valve circuit, which in turn deenergizes auxiliary fuel-valve 32, causing it to close and stopping the fuel leak through the main valve.

Energization of control-relay lCR again closes contacts lCR-l in its holding circuit and also closes contacts 1CR-3 in the leak-alarm circuit in order to signal attention to the fuel leak and to trouble with the main fuel-valve. At the same time, energization of ICR opens contacts 1CR-2 in the ignition circuit through terminals 5 and 5A, so that if heat is called for before the leak .alarm is noticed, ignition cannot take place and the burner controls lock-out as before. On the other hand, if the leak-detector is actuated while the burner is in the pre-purge period of its operating cycle, the controls will again lock-out after progressing to the point just prior to ignition. Thus, whether the leak occurs during a purge period or when the burner is in a standby condition, the auxiliary fuel-valve is closed immediately upon detection of the leak, but lock-out of the burner controls does not occur until the cycle progresses to the point just before the igniter is energized. Consequently, the fuel leak is stopped immediately, but

the blower will run through all or part of the pre-purge period, in order to flush or dilute unburned fuel, especially vaporized oil or gas, that may have been collected in the combustion chamber from the leaking main fuel-valve,

While the leak-detector is nullified by the discriminator just prior to ignition in both embodiments of the invention herein disclosed in FIGS. 2 and 4, it will be apparent that this could occur just prior to the opening of the main fuel-valve, because in both these systems preignition is provided by the gas pilot-burner 19. However, in the case of the add-on unit of FIG. 4 at least, it is desirable to energize the discriminator relay 2CR on energization of the igniter because it can then be used on systems having simultaneous ignition and fuel, as well as on those having pre-ignition. Furthermore, it could be dangerous to permit the pilot-bumer to ignite if the main fuel-valve has not been operating properly. It is also doubtful whether the advantages of keeping the leak-detector operative until the instant the main fuel-valve opens would be worth the additional expense involved in the add-on unit, at least.

It will also be appreciated that numerous devices can be employed for nullifying the action of the leakdetector, other than those shown in FIGS 2 and 4. For example, a time-delay relay can be used in place of timer contacts T14 (FIG. 2.) or discriminator relay ZCR (P16. 4). A suitable time-delay relay switch of conventional design could be wired so that it is energized on closing of operating limit-switch A, but having its switch contacts set to delay opening of the leakdetector circuit until the point in the burner start-up just before the igniter or main fuel-valve are energized. The leak-detector would therefore be operative for monitoring the system for fuel leakage up to the point where the time-delay relay contacts open just before combustion takes place. Opening of the operating limit-switch A on shutdown would then de-energize the time-delay relay and reset the leak-detector system for again monitoring fuel leakage during and following the burner shutdown cycle.

W hat is claimed is:

it. in an automatic fuel-burner system having a fuelsupply line, a main fuel-valve in said fuel-supplying line for controlling the flow of fuel to the burner, a fuel igniter for initiating a flame in a combustion chamber during each operating cycle of the fuel-bumer system and operating controls for said system having means for sequencing the operating cycle and for opening said main fuel-valve during scheduled periods in said operating cycle with the portion of said fuel-supply line upstream of said main fuel-valve under constant fuelpressure, a fuel-leak detector and safety system comprising in combination an auxiliary fuel-valve in said fuel-supply line in tandem with said main fuel-valve,

a leak-detector in said fuel-supply line downstream of said main fuel-valve operable for sensing fuel leaked by said main fuel-valve both before and. after the source of ignition is established,

control means for closing said auxiliary fuel-valve in response to the action of said leak-detector on sensing fuel in said supply-line downstream of said main fuel-valve, said control means being connected to said burner-operating controls independently of said main fuel-valve, and

discriminator means for nullifying the action of said leak-detector on said control means for said auxiliary valve during the scheduled periods when said main fuel-valve is open, such that said auxiliary fuel-valve remains open during said scheduled periods.

2. A fuel-leak detector and safety system as defined in claim 1, wherein said auxiliary fuel-valve is provided with electromechanical actuating means in a first control circuit connected to a source of power independently of the controls for said main fuel-valve,

said control means for closing said auxiliary fuelvalve comprising a switching device in a second control circuit with said leak-detector connected to the power source independently of said main-fuel valve controls,

said leak-detector comprising a fuel-sensitive switch in said second circuit arranged to close in response to the presence of fuel in said fuel-supply line downstream of said main fuel-valve,

said switching device having a first set of switch contacts in said first control circuit for closing said auxiliary fuel-valve,

said discriminator means comprising a set of normally closed timer contacts in said second control circuit co-ordinated with said sequencing means to open prior to said scheduled periods and to close following said scheduled periods, and

a lock-out device including a second set of switch contacts of said switching device in a holding circuit therefor connected to said power source independently of said burner-operating controls.

3. A fuel-leak detector and safety system as defined in claim 1, which further includes ignition disabling means responsive to said leak-detector and connected in series with said fuel igniter for preventing operation thereof when fuel is sensed by said leakdetector, said discriminator means being also effective in deactivating said ignition-disabling means during such scheduled periods.

4. The combination defined in claim 3, wherein said fuel-burner system is provided with conventional alarm systems including a flame-failure alarm and said fuelleak detector and safety system includes a leak-alarm which is activated completely independently of said conventional alarm systems in response to the action of said leak-detector on sensing fuel downstream of said main fuel-valve, said discriminator means having provision for nullit'ying the action of said leak-detector on said leak-alarm during said scheduled periods.

5. The combination defined in claim 1, wherein said fuebburner system includes an electrically powered fuel igniter and said burner-operating controls are provided with an ignition circuit for supplying electrical energy to said fuel igniter, as well as with conventional alarm systems including a flame-failure alarm and said fuel-leak detector and safety system includes an ignition-disabling switch for opening said ignition circuit and a leak-alarm, said ignition-disabling switch and leak-alarm being controlled by said leak-detector completely independently of said bumer-operating controls and of said conventional alarm system for preventing operation of said igniter and for actuating said leakalarm, respectively, when fuel is sensed by said leakdetector, said discriminator means having provision for preventing the opening of said ignition circuit and the actuation of said leak-alarm during said scheduled periods when said main fuel-valve is open.

6. A fuel-leak detector and safety system for an existing automatic fuel-burner having a main fuel-valve in a fuel-supply line for controlling the flow of fuel to the burner, a fuel igniter and operating controls for said fuel-burner having means for sequencing the operating cycle of said burner including the scheduled opening of said main fuel-valve at a predetermined point in said cycle, the portion of said fuel-supply line upstream of said main fuel-valve being under constant fuelpressure, said fuel-leak detector and safety system comprising in combination an auxiliary fuel-valve for installation in said fuelsupply line in tandem with said main fuel-valve,

a leak-detector for installation in said fuel-supply line downstream of said main fuel-valve and operable for sensing fuel leaked by said main fuel-valve both before and after the source of ignition is established,

control means for closing said auxiliary fuel-valve in response to the action of said leak-detector on sensing fuel in said supply-line downstream of said main fuel-valve, said control means having provision for being operatively connected to said burner-operating controls independently of said main fuel-valve, and

discriminator means for nullifying the action of said leak-detector on said control means during scheduled periods when said main fuel-valve is open, such that said auxiliary fuel-valve remains open during said scheduled periods.

7. A fuel-leak detector and safety system as defined in claim 6, which further includes ignition-disabling means responsive to said leak-detector for connection in series with the fuel igniter in said automatic fuelburner and said discriminator means being also effective in de-activating said ignition-disabling means during such scheduled periods.

8. A fuel-leak detector and safety system as defined in claim 7 wherein said auxiliary fuel-valve is provided with electro-mechanical actuating means in a first control circuit capable of being connected to a power source independently of the controls for the main fuelvalve in said fuel-burner, said control means for closing said auxiliary fuel-valve comprising a switching device in a second control circuit with said leak-detector capable of being connected to a power source independently of said main fuel-valve controls, said switching device having a first set of switch contacts in said first circuit for closing said auxiliary fuel-valve.

9. A fuel-leak detector and safety system as defined in claim 8, wherein said discriminator means comprises switch-means in said second control circuit and means co-ordinated with said scheduled periods when said main fuel-valve is open for opening said switch-means prior to said scheduled periods and for closing said switch-means following said scheduled periods.

10. A fuel-leak detector and safety system as defined in claim 9, wherein said switch-means comprises a device selected from the group consisting of a timer switch, a control-relay switch and a time-delay relay switch.

11. A fuel-leak detector and safety system as defined in claim 10, in which said ignition-diabling means comprises a second set of switch contacts of said switching device in the circuit to be connected in series with said fuel igniter, said second control circuit being connected in parallel with said first control circuit.

12. A fuel-leak detector and safety system as defined in claim 11, which further includes a leak-alarm circuit connected in parallel with said first control circuit, said switching device having a third set of switch contacts in said leak-alarm circuit for de-activating said leakalarm circuit during said scheduled periods.

13. A fuel-leak detector and safety system as defined in claim 12, which further includes a holding circuit for said switching device by-passing said leak-detector for locking out the operating controls for said burner, said switching device having a fourth set of said switch contacts in said holding circuit for closing said holding circuit when said switching device is energized and a manual reset switch for de-energizing said holding circuit.

14. A fuel-leak detector and safety system as defined in claim 13, wherein said auxiliary fuel-valve is open when said electro-mechanical actuating means is energized, said leak-detector comprising a nomally open fuel-sensitive switch which closes on sensing fuel in said fuel supply-line, said switching device comprising a control relay having a plurality of switch contacts comprising said first, second and third sets of switch contacts, said first and second sets of switch contacts being closed when said control relay is de-energized and said third and fourth sets of contacts being open when said control relay is de-energized, all said switch contacts being reversed when said control relay is energized, said discriminator switch means being disposed in said second control circuit between said leak-detector pressure switch and said control relay.

15. A fuel-leak detectorand safety system as defined in claim 1, wherein said fuel-burner system further includes forced-draft means, said control means for closing said auxiliary fuel-valve being also independent of said forced-draft means such that said forced-draft means remains operable for purging combustible fuelair gases from the combustion chamber. 

1. In an automatic fuel-burner system having a fuel-supply line, a main fuel-valve in said fuel-supplying line for controlling the flow of fuel to the burner, a fuel igniter for initiating a flame in a combustion chamber during each operating cycle of the fuelburner system and operating controls for said system having means for sequencing the operating cycle and for opening said main fuel-valve during scheduled periods in said operating cycle with the portion of said fuel-supply line upstream of said main fuelvalve under constant fuel-pressure, a fuel-leak detector and safety system comprising in combination an auxiliary fuel-valve in said fuel-supply line in tandem with said main fuel-valve, a leak-detector in said fuel-supply line downstream of said main fuel-valve operable for sensing fuel leaked by said main fuelvalve both before and after the source of ignition is established, control means for closing said auxiliary fuel-valve in response to the action of said leak-detector on sensing fuel in said supply-line downstream of said main fuel-valve, said control means being connected to said burner-operating controls indePendently of said main fuel-valve, and discriminator means for nullifying the action of said leakdetector on said control means for said auxiliary valve during the scheduled periods when said main fuel-valve is open, such that said auxiliary fuel-valve remains open during said scheduled periods.
 2. A fuel-leak detector and safety system as defined in claim 1, wherein said auxiliary fuel-valve is provided with electro-mechanical actuating means in a first control circuit connected to a source of power independently of the controls for said main fuel-valve, said control means for closing said auxiliary fuel-valve comprising a switching device in a second control circuit with said leak-detector connected to the power source independently of said main-fuel valve controls, said leak-detector comprising a fuel-sensitive switch in said second circuit arranged to close in response to the presence of fuel in said fuel-supply line downstream of said main fuel-valve, said switching device having a first set of switch contacts in said first control circuit for closing said auxiliary fuel-valve, said discriminator means comprising a set of normally closed timer contacts in said second control circuit co-ordinated with said sequencing means to open prior to said scheduled periods and to close following said scheduled periods, and a lock-out device including a second set of switch contacts of said switching device in a holding circuit therefor connected to said power source independently of said burner-operating controls.
 3. A fuel-leak detector and safety system as defined in claim 1, which further includes ignition disabling means responsive to said leak-detector and connected in series with said fuel igniter for preventing operation thereof when fuel is sensed by said leakdetector, said discriminator means being also effective in de-activating said ignition-disabling means during such scheduled periods.
 4. The combination defined in claim 1, wherein said fuel-burner system is provided with conventional alarm systems including a flame-failure alarm and said fuel-leak detector and safety system includes a leak-alarm which is activated completely independently of said conventional alarm systems in response to the action of said leak-detector on sensing fuel downstream of said main fuel-valve, said discriminator means having provision for nullifying the action of said leak-detector on said leak-alarm during said scheduled periods.
 5. The combination defined in claim 1, wherein said fuel-burner system includes an electrically powered fuel igniter and said burner-operating controls are provided with an ignition circuit for supplying electrical energy to said fuel igniter, as well as with conventional alarm systems including a flame-failure alarm and said fuel-leak detector and safety system includes an ignition-disabling switch for opening said ignition circuit and a leak-alarm, said ignition-disabling switch and leak-alarm being controlled by said leak-detector completely independently of said burner-operating controls and of said conventional alarm system for preventing operation of said igniter and for actuating said leak-alarm, respectively, when fuel is sensed by said leak-detector, said discriminator means having provision for preventing the opening of said ignition circuit and the actuation of said leak-alarm during said scheduled periods when said main fuel-valve is open.
 6. A fuel-leak detector and safety system for an existing automatic fuel-burner having a main fuel-valve in a fuel-supply line for controlling the flow of fuel to the burner, a fuel igniter and operating controls for said fuel-burner having means for sequencing the operating cycle of said burner including the scheduled opening of said main fuel-valve at a predetermined point in said cycle, the portion of said fuel-supply line upstream of said main fuel-valve being under constant fuel-pressure, said fuel-leak detector and safety system comprising in combination an auxilIary fuel-valve for installation in said fuel-supply line in tandem with said main fuel-valve, a leak-detector for installation in said fuel-supply line downstream of said main fuel-valve and operable for sensing fuel leaked by said main fuel-valve both before and after the source of ignition is established, control means for closing said auxiliary fuel-valve in response to the action of said leak-detector on sensing fuel in said supply-line downstream of said main fuel-valve, said control means having provision for being operatively connected to said burner-operating controls independently of said main fuel-valve, and discriminator means for nullifying the action of said leak-detector on said control means during scheduled periods when said main fuel-valve is open, such that said auxiliary fuel-valve remains open during said scheduled periods.
 7. A fuel-leak detector and safety system as defined in claim 6, which further includes ignition-disabling means responsive to said leak-detector for connection in series with the fuel igniter in said automatic fuel-burner and said discriminator means being also effective in de-activating said ignition-disabling means during such scheduled periods.
 8. A fuel-leak detector and safety system as defined in claim 7, wherein said auxiliary fuel-valve is provided with electro-mechanical actuating means in a first control circuit capable of being connected to a power source independently of the controls for the main fuel-valve in said fuel-burner, said control means for closing said auxiliary fuel-valve comprising a switching device in a second control circuit with said leak-detector capable of being connected to a power source independently of said main fuel-valve controls, said switching device having a first set of switch contacts in said first circuit for closing said auxiliary fuel-valve.
 9. A fuel-leak detector and safety system as defined in claim 8, wherein said discriminator means comprises switch-means in said second control circuit and means co-ordinated with said scheduled periods when said main fuel-valve is open for opening said switch-means prior to said scheduled periods and for closing said switch-means following said scheduled periods.
 10. A fuel-leak detector and safety system as defined in claim 9, wherein said switch-means comprises a device selected from the group consisting of a timer switch, a control-relay switch and a time-delay relay switch.
 11. A fuel-leak detector and safety system as defined in claim 10, in which said ignition-disabling means comprises a second set of switch contacts of said switching device in the circuit to be connected in series with said fuel igniter, said second control circuit being connected in parallel with said first control circuit.
 12. A fuel-leak detector and safety system as defined in claim 11, which further includes a leak-alarm circuit connected in parallel with said first control circuit, said switching device having a third set of switch contacts in said leak-alarm circuit for de-activating said leak-alarm circuit during said scheduled periods.
 13. A fuel-leak detector and safety system as defined in claim 12, which further includes a holding circuit for said switching device by-passing said leak-detector for locking out the operating controls for said burner, said switching device having a fourth set of said switch contacts in said holding circuit for closing said holding circuit when said switching device is energized and a manual reset switch for de-energizing said holding circuit.
 14. A fuel-leak detector and safety system as defined in claim 13, wherein said auxiliary fuel-valve is open when said electro-mechanical actuating means is energized, said leak-detector comprising a nomally open fuel-sensitive switch which closes on sensing fuel in said fuel supply-line, said switching device comprising a control relay having a plurality of switch contacts comprising said first, second and third sets of switch contacts, said first and second sets oF switch contacts being closed when said control relay is de-energized and said third and fourth sets of contacts being open when said control relay is de-energized, all said switch contacts being reversed when said control relay is energized, said discriminator switch means being disposed in said second control circuit between said leak-detector pressure switch and said control relay.
 15. A fuel-leak detector and safety system as defined in claim 1, wherein said fuel-burner system further includes forced-draft means, said control means for closing said auxiliary fuel-valve being also independent of said forced-draft means such that said forced-draft means remains operable for purging combustible fuel-air gases from the combustion chamber. 